This invention is a clock generator for use in a receiver for modulated signals transmitted either by radio or wire.
This invention relates to a clock regenerator or a timing pulse generator for generating, a timing pulse sequence of a timing period that is equal to a prescribed number of clock periods of a digital signal recovered in the receiver from a pertinent one of the modulated signals and phase-synchronized with the clock periods.
In digital wired communication, a transmitter comprises an encoder device responsive to sampling or clock pulses of a clock period for encoding an original or information signal into a digital signal. For transmission through a radio channel, the transmitter further comprises a modulator for modulating a carrier signal by the digital signal to produce a modulated signal to be sent through the channel. Modulated signals are also used in wired communication. A receiver for the modulated signal comprises a demodulator for reproducing or recovering the digital signal from the received modulated signal and a decoder device for decoding the recovered digital signal into a reproduction of the original signal. In order that the decoder device may properly operate, use of clock pulses which are phase-synchronized with the clock pulses used in the participant transmitter is necessary. The digital signal does not explicitly include the clock pulses. A clock regenerator is therefore indispensable in the receiver to regenerate clock pulses from the recovered digital signal and to serve also other purposes, such as shaping and/or retiming of the digital signal to be supplied to the decoder device.
A conventional clock regenerator comprises a voltage controlled oscillator for producing output pulses of a repetition period approximately equal to the clock period of a digital input signal, a phase detector for phase-detecting the input signal with reference to the output pulses to produce a detection output of a voltage dependent on the phase difference between the repetition and clock periods, and a negative feedback loop for controlling the oscillator by the detection output. The regenerator is capable of readily phase-synchronizing the output pulses with the clock periods and of achieving a high effective Q value by adjusting the loop gain to thereby appreciably raise the stability of clock regeneration against noise, jitter, and the like. The oscillator, however, follows the clock period with a considerable delay at the beginning either when the channel of the input modulated signal is switched to another channel or when the modulated signal is supplied to the receiver in bursts.
Another conventional clock regenerator comprises a differentiator for differentiating a digital input signal to produce a unipolar pulse sequence and a band-pass filter of a narrow bandwidth, such as an LC tank circuit tuned to the clock frequency or period, responsive to the pulse sequence for producing the clock pulses. Inasmuch as the amplitude of the produced clock pulses varies with the density of the clock components in the input pulses, it is necessary to use a limiter for suppressing the amplitude fluctuation. When the Q value of the filter is reduced to adapt the regenerator to the input modulated signal supplied to the receiver in bursts, the regenerated clock pulses are unavoidably subject to noise, jitter, and the like.